Quinacridone pigments are known for their attractive red and magenta colors and for their outstanding fastness properties. The method of preparing quinacridone pigments by oxidizing the correspondingly substituted 6,13-dihydroquinacridone is well known in the art.
For example, numerous publications disclose the oxidation of a 6,13-dihydroquinacridone to the corresponding quinacridone using aromatic nitro compounds as the oxidizing agent in an alcoholic medium containing a base and a small amount of water. However, such processes have the disadvantage of producing considerable organic waste due to the generation of reduced aromatic by-products.
It is also known to oxidize a 6,13-dihydroquinacridone to the corresponding quinacridone by a process wherein the 6,13-dihydroquinacridone is oxidized in a solvent and/or aqueous basic system with an oxygen-containing gas. Such processes are often referred to as “air oxidation” because air is conveniently used as the oxygen-containing gas. Air oxidation processes have the disadvantage that large gas volumes have to be introduced into a heterogeneous reaction mixture, whereby foam is generated. Additionally, it is difficult to visually determine when the reaction is complete.
Furthermore, it is known to oxidize 6,13-dihydroquinacridones dissolved in polar solvents, for example DMSO, using air as the oxidizing agent. Such processes have the advantage of generating excellent quinacridone pigments in a high yield. However, they have the disadvantage of producing a substantial amount of organic waste, such as dimethylsulfone, as by-product during the oxidation reaction, which requires costly solvent regeneration systems.
The use of hydrogen peroxide as oxidant is known and described as advantageous in that it has high oxidation efficiency at ambient pressure, is readily available and does not generate a reduced organic by-product. Specific teachings are found in U.S. Pat. Nos. 5,840,901, 5,856,488 and 6,013,127, all of which are incorporated herein by reference.